EMBRYOLOGY AND COMPARATIVE ANATOMY. OF BRAIN. 25 



there springs from the base of the fore-brain, on each side, a process 

 whose cavity at first communicates with the cavity of the ven- 

 tricles. These are the olfactory lobes. They are always cov- 

 ered with a cortex of peculiar structure, from which the non- 

 medullary fibres of the olfactory nerves arise in many bundles. 

 In the case of only a few animals (the cyclostomata, for example), 

 a single olfactory nerve springs from each olfactory lobe, and, 

 passing to the organ of smell, is divided into many branches. 

 In mammals the anterior portion of the lobes becomes separated 

 from the rest of the brain, and is known as the olfactory bulb. 

 This is connected with the lobe proper by a long, slender process 

 containing both cells and fibres, the olfactory tract. In many 

 mammals, and particularly in human beings, the olfactory lobe 

 becomes atrophic, and little more is visible on the under surface 

 of the brain than the olfactory bulb and tract. 



The inter-brain is in all animals an elongated bodv, whose 



O / ' 



lateral walls contain, in the case of the lower vertebrates, two, 

 and in higher several " thalamic ganglia." The walls thickened 

 by these ganglia reduce the third ventricle, which lies between 

 them, to the dimensions of a narrow slit. In the bony fishes 

 the mid-brain grows to such a degree that it completely covers 

 up and hides the inter-brain. The base of the inter-brain 

 bulges out to form the infundibulum, which is a large process 

 in the lower vertebrates, and which does not always become 

 fused with the hypophysis, which grows toward it from the 

 pharyngeal epithelium through the base of the skull. In many 

 fishes, particularly the selacians, numerous blood-vessels grow 

 into the infundibulum, and, pushing the epithelium before them, 

 form the "saccus vasculosus," apparently a secreting organ. 

 The roof of the inter-brain is formed anteriorly of the choroid 

 plexus. Farther back it becomes elongated into a tube, which 

 is directed forward, the tube of the epiphysis. In some sela- 

 cians, and in many reptiles, this passes through an opening in 

 the skull to an organ of special sense, which strikingly resem- 

 bles an eye. We can recognize a cornea and a lens, a retina 



